Single-side band-carrier system



Feb. 18, 1930. A CARPE 1 47:,339

SINGLE SIDE BAND CARRIER SYSTEM Filed Sept. 22, 1926 INVENTOR We Chi we A TTORNE Y Patented Feb. 18, 1930 UNETED STATES ALLEN GARPE, OF NEW YORK, N. Y., ASSIGNOR T0 AMERICAN TELEPHONE AND TELE- GRAPH COMPANY, A CDRPORATION 0F NEW YORK SINGLE-SIDE BAND-CARRIER SYSTEM Application filed September 22, 1526. Serial No. 137,13e.

This invention relates to high frequency signaling systems, and particularly to a method and means for the production of a single side band.

In high frequency signaling systems, as, for example, a carrier system in which it is desired to transmit only one side band over the system, it is customary to eliminate by means of a filter the other side band produced by the modulation of the carrier by the signal to be transmitted. E:-;cept for the loss of energy represented by the suppressed side band, this method of elimination is satisfactory where the frequencies involved are of the order now employed upon carrier systems.

As the frequencies increase, the difficulty in eliminating the unwanted side band by means of a filter becomes more pronounced, and with frequencies of the order of 1,000 kilocycles it is practically impossible to eliminate the unwanted side band by filters unless the signal currents are raised to that frequency through two or more stages of modulation with filters in each stage to eliminate the unwanted band. Such a system is, of

course, expensive to build and maintain, and there are other reasons why this practice is undesirable.

This invention resides in a method and means for discriminating against undesired modulation products by phase balance. More particularly it resides in the production of one side band only, by the direct modulation of the high frequency carrier by a low frequency signal.

Other objects of this invention will be apparent from the following descriptionwhen read in connection with the attached drawing showing schematically aform of embodiment of the invention.

In the drawing, the line L represents an input circult L is connected with two branches A and A which in turn are connected with the common output circuit L The branch A includes a balanced modulator M which is connected with the input circuit by means of the transformer 2. This modulator includes the tubes 3 and 4 which are symmetrically connected with the secondary winding of the transformer 2 and also with the primary winding of the transformer 5. An oscillator 0 is arranged to impress a voltage of a carrier frequency upon the winding 6 which is similarly connected with the grids of the tubes 3 and 4. The frequency produced by this oscillator should be such that one of the side bands appearing in the modulator output may be suppressed by means of the band filter 7 which is connected with the secondary of the transformer 5. This filter is also connected by the transformer 8 with the detector D the function of which is to detect the signal which was impressed by the line L upon the modulator M This detecting step iseflt'ected by oscillations from the source which, of course, are of the same frequency as those applied to the modulator M but differ in phase by 90 from the latter as the result of the action of the phase shifter PS inserted between the oscillator 0 and the winding 9 connected with the common conductor in the input circuits of the tubes 10 and 11 of the detector D The output side of this detector is connected by the transformer 12 with a low-pass filter 13 whose cut-off point is above the limit of the signal transmitted by the line L. This filter is connected by transformer 14 with the input side of the modulator M comprising the tubes 15 and 16 which are symmetrically connected with the windings of the transformers 14 and 17 The winding 18 in the input circuits of the tubes 15 and 16 is connected with a source of high frequency oscillations 0 the connection including a phase shifter PS whose func= tion is to shift by 90 the phase of the current applied by the oscillator to the modulator M The output side of this modulator is connected by transformers 17 with the line L. As mentioned before, the input circuit L is connected by the transformer 19 with the modulator M comprising tubes 20 and 21, which are symmetrically connected with windings of the transformers 22 and 23. The

. common conductor in the input side of this modulator is connected with the source of high frequency oscillations 0 This connection, it will be seen, includes no phase shifting device, so that the high frequency oscillations applied by 0 to the input side sate for these additional phase displacements,

a compensating phase shifter 25 is shown in the path associated with the modulator M and it is to be understood that this compensating phase shifter is so designed that its net phase displacement will be equal to that produced by those elements of the path A which are not otherwise duplicated in the path A i. e., by that portion of path A between transformer 2 and low-pass filter 13, inclusive.

The manner in which the foregoing system functions in order to produce a single side band of modulated high frequency oscillations by direct modulation of the high frequency carrier by the low frequency modulating current is as follows: The modulating frequency impressed by the source 1 upon the line L which frequency may represent speech, music or other sounds, has an instantaneous value which, for purposes of discussion, may be represented by sin pt. This will be impressed by transformer 2 upon the branch A and by the transformer 19 upon the branch A The current impressed on the branch A will be modulated by oscillations from the source O having the value sin t. Since the balanced modulator M suppresses the carrier frequency, there will be impressed by the transformer 5 upon the band filter 7 only the side bands of modulation and double frequency and unmodulated components. The side bands are represented by one represented by cos (g+p)v' and also the double frequency and'unmodulated components, so that there will be impressed by the transformer 8 upon the input side of the detector 1D only the lower side band represented by cos gp) t. There will also be impressed upon the same input circuit, but in a different manner, oscillations of the frequency 9 from the source 0 but differing in phase by 90 from the oscillations of the same frequency that were impressed upon the input side of the modulator M There will, of course, appear in the output side of the detector D the sum and the difference frequencies resulting from the detecting operation, which may be represented by cos (gp)t sin (gt+90) which equals 1 2 [sin (pt-l-90 )+sin [(2qp)t+90]] are eliminated by means of the low-pass filter l3, and the modulating frequency sin (pt+90) is-then impressed by the transformer let upon the input side of the modulator M It should be clearly understood that at the same time there is being impressed upon the modulator M by the transformer 19 the original modulating current represented by sin pt. That is to say, the modulating currents being presented to the modulators M and M differ in phase by 90. Similarly, the carriers of frequency C/Qnr which are to be modulated differ in phase by 90. The output of the modulator 1V 2 includes two side bands which may be represented by The sidebands in the output of the modulator M are represented by cos (0t,pt) cos (025+pt).

The output currents of both modulators will be impressed by the transformers 17 and 23. upon theoutput circuit L and will combine therein. The lower side bands represented by cos (ctpt) will add. The upper side band, namely, cos (ct-l-pt) produced by l 2 differs in phase from the same side band produced by M by 180, so that these side bands cancel. This leaves in the circuit L only the single side band which is represented by cos (0-19) 2,, and unmodulated and double frequency components which may be eliminated by a simple tuned circuit or filter such as 24.

It will be seen from the foregoing description that the steps performed by that portion of the circuit A between the trans formers 2 and let consist in the shifting in phase of the modulating current impressed by the line L upon the branch A The two parts of the same modulating current which differ in phase by 90 are then impressed upon modulators in which the same carrier frequency is employed, but differing in phase by 90. This results in the production of a single side band representing the modulation of a high frequency carrier by means of a lower frequency modulating current, such as speech or music. Since this high frequency carrier may be of the order of 1,000 kilocycles, it will be appreciated that its direct modulation by a low frequency modulating current representing the range of speech or music represents a substantial improvement upon systems of the prior art. To accomplish the modulation of such a high frequency in the older systems it would have been necessary to modulate a carrier of say 30,000 cycles by the speech current, and after selecting one of the side hands to modulate, by means of this side band, a second carrier of the order of 150 or 200 kilocycles. One of the side bands of this modulation would then have been selected and used to modulate the higher frequency of say 1,000 kilccycles. It will be seen, therefore, that my invention constitutes a simple method and means for the direct modulation of a high frequency by a low frequency modulating current to produce a single side band for transmission.

It will furthermore be seen that by means of the arrangement shown not only will the modulating currents be shifted in phase, but the shifting applies uniformly to all frequencies present in the two currents. Thus, in the system disclosed hereinbefore, not only have the current waves been shifted, but the shifting has been accomplished without changing the values of the component frequencies in the shifted wave. Therefore, each component frequency has the same value after the shifting has occurred as it possessed in the original wave so that in the final modulating process, there will be complete cancellation of the undesired side-band produced by the modulating currents and carrier currents that are out of phase.

It will also be understood that while phase displacements of 90 have been described above in the case both of the modulating and the carrier frequency, the desired result will be obtained by using any phase differences which will cause one side-band to be combined in the common output circuit with a net phase 'difierence of 180 or any odd multiple thereof, while the other side-band is combined at some other phase angle such that it is not cancelled or suppressed.

It will further be evident that the same general method, described herein as applied to the elimination of one side-band, may be applied for the purpose of discriminating against any unwanted product of modulation which may be balanced out by phase 0pposition in the output circuit.

While this invention has been disclosed in a particular form, it is to be understood that it is capable of embodiment in other forms without departing from the spirit and scope of the appended claims and any form of modulating devices, such as magnetic modulators, may be used.

What is claimed is:

1. The method of producing a predetermined phase difl'erence between two portions of a modulating current, which consists in modulating with one portion of said current a carrier current of difierent frequency to produce side bands of the carrier, selecting one side band, modulating with the selected side band a second carrier current of the same frequency as that used in the first modulating operation but difiering in phase therefrom by the same predetermined angular difference desired to be produced between the said portion of the modulating current, and selecting from the products of modulation the current having the same magnitude and frequency as the original modulating current but differing in phase therefrom by the predetermined phase angle.

2. The method of producing a predetermined phase difference between two portions of current the value of which is sin pt, which consists in modulating a carrier of value sin g2? with one portion of the current sin pt, selectin one of the side bands such as cos (9- 0 3 6, producing a current of the same frequency as the original modulating current sin pt, but differing from the original current by the phase angle j by beating the selected side band with a carrier sin IH- (j).

3. The method of producing a predetermined phase difference between two portions of current the value of which is sin pt, which consists in modulating a carrier sin qt with one portion of the current sin pt, selecting one of the side bands such as cos (gp)z-, producing a current of the original modulating frequency 1) but diflering therefrom by 90, by beating the selected side band with a carrier sin t-l- 90), and selecting the component sin (pt+ 90).

4. In a single side band carrier system the combination with a source of signaling current of a source of carrier current, means to modulate the carrier current by the signaling current, means to select one of the side bands resulting from modulation, a second modu- ,lating device, means to apply to the said modulating device a carrier current from the said first mentioned source, the connection 1ncluding means to shift the phase of the carrler current a predetermined angular distance, and means to select the signaling frequency current from the products of modulation, the said current diifering in phase from that produced by the original source by the predetermined phase angle by which the carrier is shifted.

In testimony whereof, I have signed my name to this specification this 20th day of September, 1926.

ALLEN CARPE. 

